The cooling rate experienced in a material being processed with a laser is an important parameter in both the design and control of such a process. Cooling rate as used herein is the rate at which the temperature at a specific point of laser beam-material interaction decreases with time following subjection to the material processing laser beam. In laser materials processes, such as welding or cutting, the settings of process parameters such as laser power, focused beam spot size and the speed at which the laser beam traverses the workpiece are determinative of the quality of the end product of the process. For example, in the case of laser welding, the nature and quality of the weld achieved by the process is determined by such process parameters. The material cooling rate experienced during processing is a direct result of such process parameter settings. Ultimately, the cooling rate is a key characteristic of the material being processed that determines whether or not the laser process is being conducted optimally.
It is therefore important to obtain cooling rate information in order to enable optimization of a particular laser materials process. As is known in the art, cooling rates are usually estimated after the fact rather than measured during the process. One known method for estimating cooling rate, performed after the laser processing is complete, is to examine the microstructure of the processed material and evaluate that microstructure in terms of phase transformation kinetics. Alternatively, or in combination with the microstructure examination method, the cooling rate may be estimated from a mathematical model of the laser process. Obviously, such estimating techniques offer no possibility for the real time control of the laser process. Instead, they only provide the ability to make an after the fact determination that the process was not conducted optimally and, as a result, the adjustment of one or more process parameters is required. Even after such parameters are adjusted to achieve optimal processing, the maintenance of the process in an optimal state during subsequent operations is only achieved through control of the process parameters to conform to previously determined settings. As a result, cooling rate control is indirect both because it is achieved through control of process parameters in accordance with predetermined settings and also because such settings are based on earlier cooling rate estimates.
It is therefore highly desirable to obtain real time cooling rate information and further to directly utilize such information for real time laser materials processing control. In the broadest sense, real time information and control are used herein to describe information relating to the process, e.g. cooling rate, obtained during process operation and the control of process parameters in accordance with the real time information, such control also being effected during process operation. It is noted that the time period between obtaining real time information and the responsive adjustments of process parameters will vary depending on the nature of the particular process. It is known in the art to use thermocouples to measure material cooling rate in real time. However, this technique of real time cooling rate measurement is limited by the thermocouple melting point, junction size and response time. It is noted that a fast measurement response time, on the order of milliseconds, is required for real time cooling rate measurement since the cooling rate experienced during laser materials processing may be on the order of 10.sup.6 .degree. C./second.
Infrared radiometric techniques of temperature measurement offer a possible method for real time cooling rate determination. Infrared radiometry as used herein is the measurement of the intensity of infrared electromagnetic radiation. Such techniques do not require contact with the object the temperature of which is being measured and therefore do not suffer the melting problem of thermocouples. The ratio pyrometer, known in the art, is one example of an infrared radiometric temperature measuring device. The ratio pyrometer measures the temperature of an object by comparing the intensity of infrared radiation emitted by the object at two different wavelengths. The computed ratio of the intensities at the respective wavelengths is proportional to the object temperature. Ratio pyrometer apparatus as presently configured and known in the art cannot, however, be used to provide real time cooling rate information. This is because, in consideration of the above noted cooling rate that may be experienced during laser processing, the ratio pyrometer is not capable of making successive temperature measurements quickly enough to provide an accurate cooling rate value.
It is a principal object of the present invention to provide a laser materials processing system including apparatus for the real time determination of cooling rate experienced on a workpiece being subjected to laser processing and the real time control of laser operation in accordance with the determined cooling rate.